Voltage level shifter circuits are used in applications where input logic voltage level signals must be translated to output signals at higher voltage levels. For example, automotive, electronic data processing, and industrial control applications require high voltage level shifter circuits to drive various peripheral devices. Such circuits are often implemented in application specific integrated circuits (ASICs) or as independently packaged circuits.
High voltage level shifter circuits translate a logic level (0 to 5 volts) input signal to signals at high voltage levels. Voltage level shifter circuits normally receive two differential input signals and output two differential output signals at different voltages than the input signals. Ideally, voltage level shifter circuits should draw no DC current from the power supplies used to determine the desired output voltages. The output voltages attainable are a function of the power supplies to the shifter circuit and the capabilities of the devices used to build the circuit.
Voltage level shifter circuits are usually implemented with logic level devices, fabricated using metal oxide semiconductor field effect transistors (MOSFETS). MOSFETs are used because they are small in size and can be easily fabricated using standard semiconductor processing techniques. With logic level signals input to a voltage level shifter circuit, the desired output voltages can be achieved so long as those output voltages are relatively low. Voltage level shifter circuits fabricated using bipolar transistors also have limitations in high voltage applications. The maximum collector to emitter voltage with the base open circuited (V.sub.BCEO) is easily exceeded on the input transistors of a standard voltage level shifter circuit, and such circuits therefore fail when high voltage levels are applied.
The semiconductor processing techniques used to manufacture circuits result in limitations to the maximum voltage ranges that may be applied across the transistor used in the circuits. For example, the typical maximum rated gate to source voltage of a transistor is limited by the gate to source oxide thickness of the transistor. For example, the typical maximum gate to source voltage of a low voltage MOSFET is 20 volts. Semiconductor processing will also limit the maximum rated gate to drain voltage of a transistor. A typical maximum gate to drain voltage of a low voltage MOSFET is 60 volts. Finally, the voltage across the drain to source of a transistor is also limited to a maximum rated voltage for that process. A typical maximum drain to source voltage for a low voltage MOSFET is 40 volts when the transistor is on and 60 volts when it is off. If the maximum drain to source voltage of a conducting transistor is exceeded, hot electron effects within the transistor will degrade the reliability of the transistor.
In many applications, however, such as where the output voltages required exceed the basic capabilities of the process used to fabricate the transistors of the shifter circuit, existing approaches are ineffective. Using low level or logic level transistors in such applications leads to overstress of these devices. As the breakdown voltages for the transistors within the voltage level shifter circuit are exceeded, the devices may fail or the reliability of the transistors will degrade. To achieve higher voltages at the output of the voltage level shifter circuit, one approach has been to make use of high voltage transistors. High voltage transistors are generally larger than the low voltage transistors, and therefore require more area on a semiconductor chip and limit the amount of circuitry within a given semiconductor chip.
Therefore, a need has arisen for an improved method and apparatus for voltage level shifting which uses low voltage transistors to achieve high level voltage shifts.